ChangeColor.java

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;
import javax.swing.*;

public class ChangeColor extends JFrame {

  private JPanel colorPanel;
  private JButton changeColorButton;

  public ChangeColor() {
    setTitle("Random Color Changer");
    setSize(300, 200);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setLayout(new BorderLayout());
    colorPanel = new JPanel();
    changeColorButton = new JButton("Change Color");
    add(colorPanel, BorderLayout.CENTER);
    add(changeColorButton, BorderLayout.SOUTH);
    changeColorButton.addActionListener(
      new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
          ColorChanger();
        }
      }
    );
  }

  private void ColorChanger() {
    Random random = new Random();
    Color randomColor = new Color(
      random.nextInt(256),
      random.nextInt(256),
      random.nextInt(256)
    );
    colorPanel.setBackground(randomColor);
  }

  public static void main(String args[]) {
    SwingUtilities.invokeLater(() -> {
      ChangeColor app = new ChangeColor();
      app.setVisible(true);
    });
  }
}